[LayoutNG] Enable DCHECK inside of NGConstraintSpace::CreateFromLayoutObject.

This also makes the function accept a LayoutBlock instead as well.

Unittests which call NGConstraintSpace::CreateFromLayoutObject on a LayoutObject
are updated to ensure that the LayoutObject is a new formatting context.

Should have no behaviour change.

Change-Id: Ibaeffaecd47b37474fc426f80b006592a3257de8
Reviewed-on: https://chromium-review.googlesource.com/c/1473393
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632815}
4 files changed